Irresistible Grace & Effectual Calling

The Bible "locates unbelief in the total inability of sinners to effect their own liberation from the bondage of the will, and ... locate faith in the unconditional election, redemption, and effectual calling of the triune God alone. God gives not only sufficient grace (that is, enough grace to enable sinners to respond positively to God if they choose to do so), but efficient grace (that is, regeneration as well as faith and repentance as gifts)."
- Michael Horton The Christian Faith, pg. 562

“You stiff-necked people, uncircumcised in heart and ears, you always resist the Holy Spirit. As your fathers did, so do you” (Acts 7:51). This passage, which is often used in an attempt to refute the biblical doctrine of irresistible grace, actually supports it. Notice the condition of the persons who are doing the resisting: their hearts and ears are uncircumcised, which is the Bible's way of saying they are unregenerate or unspiritual. A person in this condition will always resist the outward call of the gospel. The Holy Spirit may convict them of sin and work to show them their need to Christ, but as long as they remain unregenerate, their hearts will remain closed to Christ. Irresistible grace does not mean that whenever the Spirit works He is irresistible. Rather, it means that while His promptings are always resisted by the dead in sin, He can make the gospel irresistible when He opens their spiritually blind eyes, when he opens their deaf ears and turns their heart of stone to a heart of flesh. He quickens us while we are dead, which is no work of man. As Ephesians 2:5 says, “Even when we were dead in our trespasses, [He] made us alive together with Christ--by grace you have been saved.”
Monergism.com

All that the Father gives Me will come to Me, and the one who comes to Me I will by no means cast out. For I have come down from heaven, not to do My own will, but the will of Him who sent Me. This is the will of the Father who sent Me, that of all He has given Me I should lose nothing, but should raise it up at the last day...No man can come to me, except the Father which hath sent me draw him, and I will raise him up at the last day.
John 6:37-39 and 44

Related: Regeneration, Ordo Salutis